# identifying ferrite rod with high permeability

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#### mik3ca

##### Member
Before I begin winding my coils around rods for wireless power, I was using online calculators to calculate inductance because in my other thread, someone recommend the inductance should be in the order of millihenries.

Here's one calculator: https://rimstar.org/science_electronics_projects/coil_design_inductance.htm

If a rod has a permeability of 20, then I'll need to make a lot of turns in my coil to achieve millihenries inductance.

If all other parameters are the same and permeability is a high number, then the inductance may even reach henries.

I'm looking at a bunch of rods to buy from ebay due to their price and some sellers suggested some are used for radio.

I could also visit a store but they could charge me alot of money just for a single rod.

Is there a way to identify a permeability of a rod?

#### Nigel Goodwin

##### Super Moderator
Perhaps you should try reading the other thread - you don't want ferrite rods, for the power coupling to work you must have air cored coils.

Try studying the link I posted for the low cost kit power coupler.

#### mik3ca

##### Member
air then means more windings tho because research tells me air permeability is 1

#### ronsimpson

##### Well-Known Member
Most of the wireless power I have seen use air. Many turns! Nigel knows many things. If most people use air you should to.

I did wireless power using a "transformer" like this.
Red wire on one side. Blue wire on the other side. Green is "flux" in the air.
The gap was small. And the "E" cores must be aligned just right. (if the E is rotated 90 then there will be no power)
Also this core is very fragile.

#### Nigel Goodwin

##### Super Moderator
air then means more windings tho because research tells me air permeability is 1

Feel free to wind your coils on ferrite rods, but it WON'T WORK!!

You've seen various examples on the net, you've even built one - but completely ignored the coil design, which was why it didn't work.

You're not winding RF chokes, you're effectively winding the two halves of an air cored transformer, it won't work if you don't do it right.

I'm tending towards the belief that you're nothing but a troll, continually posting utter rubbish and totally ignoring all advice given, just to gratify your own self importance.

#### alec_t

##### Well-Known Member
You could make two air-cored coils then put them both on a shared ferrite rod to improve the magnetic coupling between them.

#### mik3ca

##### Member
Feel free to wind your coils on ferrite rods, but it WON'T WORK!!
Now I'm completely confused. Audioguru said to me that by wrapping the coils around the ferrite rod increases inductance and changes inductance so that nothing would overheat as much, and now others are telling me to use air core. Are the rods supposed to be useless then?

You've seen various examples on the net, you've even built one - but completely ignored the coil design, which was why it didn't work.
I actually did the 30 turns and many examples were different variations. One even did a coil by making a long PCB trace into a spiral, but the problem is no one is technically explaining to me in great detail (maybe except audioguru) why one variation works best over all others.

You're not winding RF chokes, you're effectively winding the two halves of an air cored transformer, it won't work if you don't do it right.
That's why I tried 30 turns instead of a small fixed inductor

I'm tending towards the belief that you're nothing but a troll, continually posting utter rubbish and totally ignoring all advice given, just to gratify your own self importance.

I'm not a troll, I'm just confused.

I'll explain...

Say there was a simple light circuit consisting of an LED, a resistor, and a 9V battery and the objective was to turn the light on and someone didn't know how to do it. Say he got the response of this:

"turn battery so little circle sticking out of the battery is north. then take the peanut thing with wires sticking out and twist it together with one pin of the LED. if the other pin leads to the end of the LED with a flat end, then touch that pin to the big ring of the battery. and the other pin to the small ring. If not, then reverse battery connections"

The problem with that response is that there's no technical valid reasoning. This response would be better:

"connect + side of battery to anode (long pin) of LED then short pin of LED connects to resistor, then resistor connects to - side of battery. connecting LED in reverse will make it not work."

#### dr pepper

##### Well-Known Member
Commercial 'phone chargers use ferrite, I had a look inside one, but the coil is a pancake coil not a stick, a am aerial type loopstick will have a ui or permeability of about 3000, but it wont be a fat lot of use for wireless power as the magnetic field will be concentrated at the smaller diameter not so much across the width, I'm not an expert in the field however the larger the face area the better.
I've had success with wireless power as you've seen in your other post, 50ma or so is easy, an amp wouldnt be so hard.
If you really want to use ferrite maybe you could find a circular ferrite core smps transformer and use it in its 2 halfs with a winding on each, if you can get them a mm or so close togther you stand a chance of decent power transfer.
One difficulty with higher power is that the core will tend to saturate when its mate is not in range, as forcing the magnetic field to go through the air when the transformer is split will massively lower its inductance, a self resonant design is probably the way to go.
To get a rough idea of permeability wind a 100 turn coil, measure its inductance, then put it over the ferrite, measure the inductance again, divide the latter by the former to get rough permeability, this is only ball park.

#### MrAl

##### Well-Known Member
air then means more windings tho because research tells me air permeability is 1

Hi,

As a rule the most important thing about a magnetic circuit is distance which in a magnetic circuit means length.
The next most important thing is the magnetic circuit itself, in that the flux is said to circulate similar to the way an electrical current circulates in an electrical circuit.
This means to understand your setup you have to figure out what the magnetic circuit is and what the circulation path lengths are.

For a magnetic circuit that is made from all core material and no air, the permeability is just about the permeability of the core because the flux is always through a magnetically active material with high permeability relative to air. For a mag circuit that goes partly though air though, the decrease in permeability is abrupt and very significant. Even a small gap 0.1 inches can have a profound effect on the permeability, and the permeability is what increases the self inductance of an air coil when inserted in the space where the air was originally.

The problem with a 'rod' is that the air gap is as long as the core itself. That big of an air gap will make the construction look only a little better than air. The other problem is that it is hard to complete the magnetic circuit which must start at one end directed outward and end at the other end directed inward. Whatever you use as a receiver has to be able to be part of that circuit, and because one end of the rod will be very far from the receiver it will make a very poor magnetic path.

The idea is to keep the entire magnetic path as short as possible, with emphasis on the shortness of the air part of the path. So if you look at different geometries you see different results. A round flatter coil works better because the turns at the back of the coil are not that much farther from the receiver than the turns at the front of the coil.

I am not sure if you should get too hung up on the value of the self inductance. There will be an optimum value yes but it may be hard to get.

What you could try if you feel you need to use a rod is use two rods, one for the transmitter and one for the receiver. Keep the two rods parallel and as close together as possible.

Take a look at ron simpson's post and see how that setup words, then compare it to the two rods. Note the difference in the air gaps, which makes the most difference in power transfer. The other idea is to get the mutual inductance as high as possible, and the smallest air gap(s) achieves that.

#### audioguru

##### Well-Known Member
The ad for the Chinese wireless charger showed flat coils with no core and said that the coils were tuned to resonance. The air gap between coils was very small, a piece of paper or a thin plastic housing.

##### Well-Known Member
Before I begin winding my coils around rods for wireless power
How much power? A Google of Electric Toothbrush Chargers will bring up dozens of low power circuits. Additionally a Google of Inductive Charging Circuits should provide some ideas. The guy with some good ideas on this subject was Nikola Tesla who unfortunately is no longer with us.

Ron

#### ronsimpson

##### Well-Known Member
The toothbrush I took apart; the battery charging coil is inside the transmitting coil. The coupling is probably real good.

##### Well-Known Member
The toothbrush I took apart; the battery charging coil is inside the transmitting coil. The coupling is probably real good.
I probably should not post this but I have an Oral B electric toothbrush and that sucker has been going for about 10 years now. Just watch, tomorrow morning it won't make it through a brushing.

Ron

#### dr pepper

##### Well-Known Member
Practical electronics did a project for a marine xenon strobe back in the 80's, it was inductively charged to maintain water tightness, the article might be online.

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